1. Field of the Invention
The present invention relates to a multiple path optical matrix system and, more particularly, to a multiple reflection optical instrument used for the component analysis of a gas or the like and a reflected light catching method using the same.
2. Description of the Prior Art
In a conventional technique of this type, a laser beam is caused to pass through the housing of a cell in which a gas to be analyzed is charged and the wavelength intensity of the exit light beam is analyzed, thereby performing the component analysis of the gas. In order to perform this analysis, it is necessary to realize an optical path (a transmission path) as long as possible in the housing.
In a classic system developed by White, a plurality of mirrors are used to prolong the optical path. However, this system cannot provide a sufficiently long optical path.
A technique is recently disclosed in U.S. Pat. No. 4,626,078 to Chernin et al. This technique relates to a multiple path optical matrix system having an arrangement schematically shown in FIG. 1 and an operation described below with reference to FIG. 2.
Referring to FIG. 1, this system is constituted by a cylindrical cell having a housing 4. A main field mirror 24 and an auxiliary field mirror 25 adjacent to the main field mirror 24 are arranged at one longitudinal end of the housing 4. On the other hand, a set of four pieces of objective mirrors 39, 40, 41, and 42 having the same radius of curvature are arranged on a mount 34 disposed at the other longitudinal end of the housing 4 opposite to the field mirrors 24 and 25. The main field mirror 24 has the same curvature as that of the objective mirrors. The center of curvature of the main field mirror 24 matches the center of symmetry of the four pieces of objective mirrors along the longitudinal axis of the housing 4. In the multiple reflection optical apparatus having the above arrangement, a light beam from a light source is incident in the housing 4 through an inlet window 2 provided on the field mirrors side. The light beam is repeatedly reflected between one of the objective mirrors 39 to 42 and one of the field mirrors 24 and 25 a plurality of times. The light beam beam finally emerges through an outlet window 27 formed on the side of the inlet window 2, and the wavelength intensity of the light beam emerging from the outlet window 27 is measured. The reflection operation in the housing 4 of the above multiple reflection system will be described below in relation to an image matrix shown in FIG. 2.
The light beam emitted from the light source passes through the inlet window 2 formed at one longitudinal end of the housing 4 formed in the cell. The light beam is directed to the first objective mirror 39 of the four pieces of objective mirrors and reflected thereby toward the main field mirror 24 to perform the first coming and going reflection in the housing 4. A first spot 1 is located at one corner of the image matrix focused on the main field mirror 24. The light beam is directed from the first spot 1 on the main field mirror 24 to the second objective mirror 40, reflected thereby, and directed to the first objective mirror 39 once more through the main field mirror 24. The reflection between the first objective mirror 39 and the field mirror 24 and the reflection between the second objective mirror 40 and the field mirror 24 are alternately repeated a plurality of times until the reflected light beam reaches a seventh spot 7 focused at a cross poit of one end column of the image matrix on the main field mirror 24 which includes the first spot 1 and the lowermost line thereof. At this time, the light beam is incident through the first objective mirror 39 on the auxiliary field mirror 25 provided adjacent to the main field mirror 24 on the inlet window 2 side and having the same curvature as in the objective mirrors and the center of curvature matching the point of symmetry of the first and third objective mirrors 39 and 41. The light beam reflected by the auxiliary field mirror 25 is directed to the third objective mirror 41, incident on the main field mirror 24 again through the third objective mirror 41, and directed to the last fourth objective mirror 42.
After repeating this reflection a plurality of times, the light beam is finally reflected by the first objective mirror 39 in the housing 4, emerges from the housing 4 through the outlet window 27 including a spot corresponding to an odd-numbered coming and going reflection (45th coming and going reflection in FIG. 2) in the housing 4, and is catched by a sensor or the like.
Practically, however, when the above conventional system is used in an environment, e.g., in a running car or flying airplane, which produces a very strong vibration, the cell of the multiple reflection optical instrument is distorted to cause variation of the optical path. This makes it impossible to stably catch the transmitted light beam at the outlet window. This phenomenon will be described below in relation to FIG. 3.
Assume that, during use of the conventional system, distortion of the cell by the vibration causes one mirror M of the field mirrors or the objective mirrors to tilt by an angle .DELTA..alpha.. At this time, a light beam 1 directed to the mirror M makes a light beam 1b reflected from the mirror M, which is shifted by an angle 2.DELTA..alpha. with respect to a reflected light beam 1a in a normal state. That is, the reflected light beam 1b is shifted by an angle twice the tilting angle of the mirror by distortion of the cell. Therefore, the light beam finally reflected in the housing can be greatly shifted from the outlet window depending on the magnitude of distortion of the cell and the length of the housing to make it impossible to catch the light beam. It can be considered to anticipate the maximum distortion of the cell and form an outlet window having an opening area large enough to cope with this shift. However, the size of the cell itself is limited. In addition, when an optical sensor is used in accordance with such a large opening area, the accuracy of the sensor is greatly degraded, and the object to perform the gas analysis or the like cannot be achieved.
Further, when air is sampled in a flying airplane while moving at a high speed, it is difficult to repeat measuring in a specific spot. Therefore, the light beam must be caught without any error.